Parts that are used on naval ships must meet high quality standards and preferably be as light in weight as possible. Certain structural shapes are typically fabricated by such methods as deflanging or splitting hot-rolled I-beams. Use of hot-rolled sections has certain inherent disadvantages with regard to product quality. For one, the tolerance range is wide, requiring that many parts fabricated from these shapes be reworked. For example, hot-rolled I-beams are often split in two to obtain T-beams. The position of the flange relative to the web varies widely on hot-rolled I-beams, which may require that the T-beam be reworked. For another, the distortion on the parts resulting from working the material can be unacceptably great, which, again, requires that the part be reworked or compensatory measures taken when the hot-rolled section is assembled. Reworking of parts increases considerably cost and fabrication time of parts.
In addition to the disadvantages mentioned above, the use of hot-rolled sections limits the designer to certain sizes and shapes, thereby resulting in a higher weight product. Hot-rolled structural parts are made in standard sizes and shapes, with large jumps in dimensions between sizes. The structural requirements of a part may allow the design of a part that has dimensions that lie between two sizes of conventional parts. Since it is not acceptable to use a part with strength and rigidity properties that are lower than specified, the larger size must be used, resulting in a part that is dimensionally greater and stronger than necessary. The difference in dimensions most certainly adds unnecessary weight to the structure.
The use of hot-rolled structural sections also limits the types of material used. Most hot-rolled structural shapes or parts for shipbuilding are made from AH36 material. Higher strength steels (HSLA 65, 80, or 100) are available in plate stock only. The use of these higher strength steels would allow the structural parts to be designed to smaller dimensions, which would result in a significant reduction in weight.
The conventional deck on a ship is constructed from steel plates, with T-beams welded to the plates to provide additional stiffness. With conventional welding systems, it is difficult to control dimensional tolerancing and distortion. Secondary processes are then applied to the welded structure to eliminate the distortion. This is typically done by applying heat to the distorted parts, which may result in higher residual stresses in the parts. The necessity of applying these secondary processes raises the cost of the structure in terms of both time and cost.
It is possible to construct the deck as a composite structure, using two much thinner steel plates of high strength steel, with a corrugated structure sandwiched between them, then welding T-beams to the structure to add stiffness. It is estimated that the use of such a composite construction for the decks of a ship would result in up to 50% overall weight savings of the ship. The difficulty today with employing such construction methods lies in the seam tracking and weld quality control systems. Conventional laser-welding systems that could weld a T-beam do not have a comprehensive process control and integrated weld quality inspection system that provides feedback and weld control during the weld process. It is known in laser welding systems to use a seam tracker to guide the laser beam along the weld seam, but the inspection systems are not coupled into the weld control processes. In other words, they don't feed data back to control the process. Such control could correct processes that are going off-specification, before it is too late and a deficient weld has been completed. With conventional laser welding systems, the inspection is a separate offline process, that is, the welded parts are transported to an inspection station offline, where the final weld inspection and secondary processes are applied.
The difficulty of controlling the quality of a weld is magnified when welding large structural parts. With complex three-dimensional shapes, the errors are also three-dimensional and are accordingly very difficult to properly measure and correct.
What is needed, therefore, is a system for and a method of fabricating structural parts that significantly reduce the amount of rework. What is further needed are such a method and a system for fabricating such structural parts from plate stock, rather than hot-rolled sections. What is yet further needed is such a system that enables integration of the weld process with other systems.